Definitely go to the two Six Nations matches. They are competitive games, whereas the first is a friendly. The atmosphere will be better at the other matches.
I agree with the above, the 6Ns games are the best. But if you want to see the Australia game then the England game is the best one in the 6Ns!
I would go to see Wales vs Ireland if you are a Welsh fan. That should be a more interesting game..............I think anyway.